Sorry, I had it partially wrong. There‘s the C++ Interpreter and the baseline interpreter (that one was added only recently[1]), and then the two JITs (BaselineJIT and IonMonkey). I understand that IonMonkey itself has two levels chosen depending on code hotness.

I.e. these settings will kill all JITs (so the highest 2-3 tiers) and leave the two interpreters.

[1] https://hacks.mozilla.org/2019/08/the-baseline-interpreter-a...